(I2)n encapsulation inside TiO2:: A way to tune photoactivity in the visible region

We report on the synthesis of nanovoid-structured TiO2 material via a sol-gel route using titanium isopropoxide as precursor. The nanovoids are formed during the thermal treatment in air at 773 K. The surfaces of internal cavities are populated by the partial oxidation products of the organic part of the Ti precursor (CO2, hydrogen carbonates, and residual isopropoxide groups). The thermal treatment in air at 773 K allows the maintainence, in the internal voids, of the encapsulated species. Addition of iodine in the synthesis procedure results in a new nanovoid-structured titanium oxide able to absorb light in the whole visible part of the electromagnetic spectrum. The origin of this absorption is attributed to the presence of (I-2)(n) adducts encapsulated in the nanocavities. These species coexist with partial combustion products of isopropoxide groups. Due to the protection of the TiO2 walls, the (I-2) n adducts are not destroyed by thermal treatments in air. We have investigated whether the electron promoted in the excited state of the dye* molecule (upon absorption of visible light from the (I-2)(n) adducts) can be injected into either the TiO2 conduction band or some titanium-localized acceptor, followed by migration of the injected electron to the surface where it reduces adsorbed organic molecules. Preliminarily experiments conducted with sunlight show that the surface-specific efficiency of this process, tested by following the degradation of methylene blue, is about 10 times higher than that of the P25 commercial TiO2 photocatalyst.